1. Principles
of the Barkhausen Noise method
The Barkhausen
Noise method can be used to detect residual stress only on magnetic ferrous
materials (iron, cobalt, nickel and relative alloys). As known, the magnetic
domain in these materials are generally casually oriented (see sketch a).
Under these conditions the global magnetisation is null. The application of
a magnetic field, or a mechanical deformation, causes a change in the structure
of the domains (movement of the boundaries that separate them or rotation
of the magnetising vectors) which in turn modifies the global magnetisation
of the piece.
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Particularly for materials with constant positive magneto-striction (iron and cobalt), traction in one direction favours orientation of the domains in the same direction (sketch b), while compression favours orientation in a perpendicular direction (sketch c). Application of the magnetic field again tends to orient the domains in the direction of the field (sketch d) summing its own effect to that of traction and subtracting it from that of compression when applied in the same direction.
